Process of burning lime and apparatus therefor.



H. L. DOHERTY.

APPLICATION FILED SEPT. 14, 1908.

Patented Nov. 2, 1909.

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HENRY L. DOHERTY, OF NEW YORK, N. Y.

PROCESS OF BURNING LIME AND APPARATUS THEREFOR.

Speciflcatien of Letters Patent.

7 Patented Nov. 2, 190a.

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To all whom it may concern:

Be it known that I, HENRY L. DoHER'rY, a citizen of the United States,and resident of New York city, in the county of New York andState of NewYork, have invented certain new and useful Improvements in Processes ofBurning Lime and Apparatus Therefor, of which the following is aspecification.

My invention relates to aprocess for burning lime and to apparatustherefor.

It relates, particularly, to that kind of process and apparatus whereinthe heat for burning the lime is derived from the com-' bustion ofproducer gas. 7

The objects of my invention are, the furnishing of a method and meanswhereby more of the heat of the kiln can be recuper-.

ated than is at present possible in apparatus of the class mentioned,the avoidance of the danger of over-burning the lime by the regulationof the temperature in the combustion zone of the kiln, and the expansionof this combustion or high temperature zone to occup; a much greaterspace in the kiln.

- y invention consists, briefly, of a limekiln in which the lower partor cooler of the'kiln is deeper than is, at present, customary,-a gasproducer appurtenant to the kiln, a second and outer s ell surroundingthe main shell of the kiln and forming with said shell an open annularair space, said annular space having air inlets around its periphery atthe top and connections from the bottom through a fan with the ash pitof the producer, air inlets at the lower part of the cooler, a fan forthe purpose of introducing kiln gas into the ash it of the producertogether with the air be ore mentioned and another fan for the purposeof Introducing another portion of the kiln gas mto the producer gas forthe fpurpose of so moderating the temperature the flame of the burn1 nggas that the danger of over-burning 1s avoided, and the process ofburnin lime by means of a producer gas so modifi by the aforementionedapparatus. 1

In the drawing I have shown across-section of the apparatus in amoreor-less-diagrammatic form, the two blowers being in elevation.

Figure 1 is a section through the kiln and producer on a vertical planethrou h their axis, the blowers being shown in e ovation. Fig. 2 is afront elevation of the lower part of the cooler showing the doors forthe disoperation in the manner wellknown to thosev charge of the limeand the ports for the inlet;

of the air for the combustion of the pre ducer gas.

1,'refers to the lime-kiln proper; 2 is the gas fproducer; 3 is theblower supplying the dra current to the producer; and 1 is the blowersupplying the kiln gas to the pro ducer gas.

5 is the lower portion of the kiln, commonly called the cooler; 6designates the zone of the kiln in which the stone is dissociated orburned; and 7 designates the upper portion of the kiln Whose function issimply to insure suflicient time of contact between the stone and hotgases from the dissociation zone to enable the stone to take up from thegases all the heat which it is capable of absorbing.

8 is t e charging opening of the kiln, closed by a door or cover, 9.

10 is an annular air space formed between the shell proper of the kilnand the outer shell, 12. Aroundthe upper part of this jacket arearranged ports, 13, controlled by the gates or dampers, 14.

is the waste gas stack of the kiln, havinga gate, 16; 17 is a pipe orconduit conveying a portion of the kiln gas to the suction pipes 18 and19 of the fans or blowers 3 and 4, respectively; 20 and 21 are gates onthe respective suction pipes 18 and 19; 22 is a pipe connecting thelower part of the air jacket 10 with the suction pipe 18 of blower 3,having a gate, 23.

24 is the discharge pipe of blower 3, and 25 that of blower 4.

26 is the producer gas conduit leading off from the producerandconnected with a bustle-pipe, 27, encircling the kiln. This bustle-pipe,27, has connections, 28, into the shaft, 6, of'the kiln.

29 is the metallic hopper in which the cooler, 5, terminates. Thishopper is provided with doors, 30, operated in any con venient manner,and air inlets, 31. The air inlets are so designed that their free areamay be regulated at will.

32, 32, etc., are ports passing through the kiln walls, and permitting.the ignition of the gas when the kiln is started in operation and thebarring down of the lime. 33 are plugs closing these ports; 34 aresuitable liallges or dampers on the gas inlets into the The method ofoperation is as follows: The gas producer having been brought intoskilled in the art of producer operation, the producer gas which, atfirst, is allowed to escape into the atmosphere until the producer is innormal operating condition, is-

immediately upon its entrance into the shaft,

of the kiln and burns. The air for its combustion (which I. willhereafter call the secondary air) is admitted through the damper 31 atthebottom of the cooler. When the gas flame has been maintained for asuf-- cient len h of time to thoroughly heat the interior 0 the kilnfor, say, ten or fifteen feet above the ports 28 to full redness,limestone is graduallyi charged into the kiln. After the level of thestone has been brought up to within a short distance of the gas ports itshould be charged in small portions so as to avoid reducing thetemperature of the gas flame below the point of ignition. When a body ofhighly heated stone (it should be, at least, at a full red heat) hasbeen built up. for a distance of six to ten .feet above the gas ports,the remainder of the shaft can be filled as rapidly as may be desired.As the calcination of the stone in the vicinity of the gas portsproceeds, its temperature becomes hlgher and higher since the heatdeveloped by the gas flame goes more andmore to raising the temperatureof the calcined stone (lime) and less and less is rendered latent in thedissociation of the stone. If this action proceeds unchecked thetemperature of the lime finally reaches a point at which the impurities,silica, alumina, etc., which are present in greater or less amount inall limestones, will unite chemically with part of the lime (CaO)present. This causes a partial fluxing of the lime with the roduction ofa roduct which will slake wit difliculty, and, 1n large part, not atall.

In practical lime burning it is the aim to draw the lime from the bottomof the kiln at such a rate that the lime is removed from the influenceof the high temperature flame just before the calcination is complete.As mentioned above, during the operation of calcination the tem eratureof the material is'maintained consi erabl below the tem erature of theflame with w ich it is bathe on account of the fact that a large portionof the heat taken up by the stone is transformed into latent heatthrough the dissociating reaction incident to llme burning. Thisreaction-takes lace according to either one or both of the ollowingequations Reaction 1 is the one which takes place limit.

when calcite, marble or any carbonate rock in which the base is calciumis subjected to heating at or above redness. Reaction-2&5

heated. Usually the so-called limestone" is neither .a pure calciumcarbonate rock nor a the one which occurs when dolomite is pure dolomitebut of a composition between the two.

By reaction 1, reducing the datatostand- A ard conditions of temperatureand pressure,

about 778 B. T. U. are absorbed erpound'.

of stone calcined. If we take t e specific heat of limestone as .22,this heat rendered latent would have been sufficient withoutdissociation to have 'raised the stone to a temperature of .22

cut, therefore, that if the limecould be withdrawn from the influence oftheflame immediatelyon the completion of the above re- E=3536 F. Itisappar action there would be no danger of overburning. In ractice,however, with the customary met Ode of burning, it is, for

various reasons, very diflicult'toso regulate the operation of thekiln'so as to even approximately secure the desired end. The

lime produced is therefore, usually, of an. I

irregular uality. Besides,.grea t skill is required of t e men operatingthe kiln. Now, by t-he method of operation which 1 herein describe andclaim all danger-of over-burning is done away with, and, at the sametime, a great saving effected in fuel consumption owing to the recueration of the heat ordinarily carried out 1n the lime and the saving inradlatlon losses which I effect. I secure my temperature control in thekiln by mixing with the producer gas generated in the producer 2, moreor less gas withdrawn from the upper portion of the kiln. The heatliberated in thecombustion of the gas must therefore go to raising thetemperature of this portion of kiln as so introduced as well as to thecalcinationof the stone and the raising of the temperature of the stone,air and calcination products. By regulating theproporti'on of kiln gasesto producer as it is manifest, that I can reduce the initial temperatureof the producer gas to any point-within the ignition It is my object,however, to add simply-a sufficient quantity of kiln gases to reduce thetemperature of the flame to a point at which there' is no dangerofoverburning the lime. The actual proportion of kiln gas added to theproducer gas depends upon the conditions in the kiln at any given timeand should be regulated from time to time accordin to the indicationsgiven by the actual con tions prevailing in the kiln.

For example, where it is the practice to draw the lime at comparativelylong intervals,- (say-6 hours) during the drawing and'for some timeafterward the kiln gas should be cut to a minimum or omitted altogether.

This is on account of the fact, that, the withdrawal of the lime and theconsequent settling-of the column of material in the kiln carries'intothe combustion or high temperature zone a large mass of material whichis at a temperature very much below the normal temperature in that zone.The heat absorbed from the flame by the stone is therefore much morerapid than that which normally takes place at this zone. Asthetemperature of the stone increases to the normal temperatore of thecalcining stone in this zone of the kiln, I gradually increase theproportion of kiln gas until normal conditions have been reestablished.\Vith gas of normal composition I find that the volume of kiln gas addedto the producer gas as diluent should be about equal to double that ofthe producer gas. As I have previously stated, however, the proportionmust be varied to meet the local conditions, which are seldom exactlyidentical at any two plants. For example, one plantmay be calcining astone which is a comparatively pure calcium carbonate, where another maybe working on a dolomite rock. In such case a hi h-flame temperaturewould be dc mand ed in the first plant while only a moderate one wouldbe necessary in thesecond. I would therefore use less kiln gas with theproducer gas in the first case than in the second.

The kiln gas withdrawn from the upper part of the kiln and added againat the combustion zone acts simply as a carrier of heat. It takes up inthecombustion zone the heat which would otherwise go to raising thetemperature of the calcining and calcined stone to a dangerous degreeand yieldsit up again to the cooler material above the calcining zone.Its action is therefore to keep a large column of the material at a safecalcining temperature while preventing a dangerously high temperaturedeveloping in the combustion zone. In other words,

while limiting the maximum temperature.

in the. kiln, I greatly increase the mean effective temperature in thecalcining portion. Owing to this expansion of the high temperature (11.c. calcining) zone, I find it advisable to considerably increase theheight of my kiln above the dimensions that usually obtain. For example,if a. height of 40 feet above the gas inlet ports gives the best resultsin a kiln working with the ordinary method, to give the best results bymy method of burning the height should be increased to say 55 feetabove. the gas inlets. The secondary air enters through the dainpers 3]in the bottom of the cooler 5, and

rises through the interstices of the column of lime occupying thecooler. This lime passes out of the combustion zoneat very nearly thetemperature, therein. Partjof the heatwhich it carries is absorbed inthe .utilize the heat of the lime.

elimination of the last portions of carbon dioXid, which is usually notcompletely eliminated in the calcining zone proper. A large portion ofthe heat which the lime carries out of the calcining zone is, however,carried down into the cooler 5 as sensible heat, and yielded up by thelime to the secondary air passing up through the cooler.

In the application to lime burning of producer gas firing by methodsheretofore used it has been found impossible to secure an output perunit weight of fuel consumed of more than about 55% of that secured bythe earlier methods of burning the fuel in direct contact with thestone. This is due to the fact that the early lime burners, more as amatter of convenience than for any intelligent appreciation of thebenefits to be secured thereby, entered air which was to burn the fuelmixed with the stone, at the bottom of the kiln. The entering cold airtook up the heat of the hot lime and thus restored it to the calciningor combustion space, Now, when producer gas firing was applied to limeburning the importance of this heat .recupcration in the old style kilnswas apparently overlooked. At any rate, it has been the universalcustom, where producer firing has been used, to introduce the air forthe combustion of the gas at the same section of the kiln as that atwhich the gas was introduced. Indeed, by ordinary methods of produceroperation, it is not possible to fully In this case it is necessary topass about one-half of the theoretical air required for the combustionof the fuel through the gas producer itself. Now the weight specificheat of air and lime is very nearly the same (about .22 in each case).The best results heretofore obtained' in lime burning have been about 8lb. .of lime to 1 lb. fuel. One pound of carbon requires, theoretically,eleven and one-half pounds of air for its combustion. The fulltheoretical heat capacity of the air, there? fore, in the old style oflime burning was only about one and'one-third times that of the lime. Ifnow we would try to take up the heat of the quantity of lime mentionedabove in a producer gas fired kiln of the type at present used by theair for the combustion of the gas we would immediately perceive that theheat'capacity of the portion of the air which we could enter at thebottom of the kiln would be only about twothirds of that of the lime. Bythepresent methods, therefore, it would not be possible to return to thekiln all of the heat of the lime and even the improvement on presentmethods of entering the air for the combustion of the gas at the bottomof the kiln would not permit/of as high an output per to combustion, butthis would result in no saving since the excess of air would cause to becarried out at the top rather more than the heat it would take up in thelower part. Now, one of the principal objects of .my

present invention is to make possible such a N ow, this reaction is ahighly endothermic or heat absorbing reaction, the net heat absorptionbeing about 6700 B. T. U. per lb. Y

of carbon consumed. I have utilized this reaction in the processrevealed in my Letters Patent 829,105, dated Aug. 21, 1906, for thepurpose of regulating the temperature of my producer, In this presentinvention this object is only corollary to my main object, which is toburn as large a proportion of the fuel in the producer as possible bythe available oxygen of carbon dioxid. For this reason, I introduce theair and the kiln. gas bearing the carbon dioxid into the producer at thehighest practicable temperature. In

other words, I aim to maintain the temperature of 'my fuel bed, so faras possible, by the sensible heat of the gaseous current introduced,rather than by combustion of the fuel by air.

It is evident that for every 3.67 lb. of carbon dioxid which I candissociate in my producer I can dispense with a weight'of air carrying1.33 lb. of oxygen or 5.77 lb.-of

air. At the same time Ihave increased the demand for air in thesecondary combustion by an equal amount due to the extra volume ofcarbon monoxid roduced from the carbon ofthecarbondioxid issociated.'Asaresultof my improved method of operation,therefore, I transfer aportion of the volume of rimary air-ordinarily passed into the pro ucerto the secondary air which I enter at the botton of the kiln. Since I amcontinuously drawing off from the kiln a volume of gas corresponding tothat which I introduce into the producer, I do not in any way increasethe loss of heat in the gases passing out of the kiln.

, In order to introduce the greatest possible quantity of heat in thedraft current supplied to the producer, I heat the primary air-as shownin the drawing by drawing it from an annular air jacket enveloping the 7shell of the kiln. The air is admitted l through dampers at the top ofthe jacket so as to insure a proper circulation of the air through theannular space. The ,circula tion of the air through the jacket beingcomparatively slow, owing to the relatively small quantity of primaryair used in my method of the air comes into play. That 1s, the air spaceacts in a measure as it would if it were a strictly closed air jacket.The heat not beingwithdrawn as rapidly from the shell of the kiln aswhen the latter is freely exposed to the atmosphere accumulates, so to seak, in the metal shell, raising the latter an theair in contact with itto a much higher temperature than the normal temperature of the shell'when the'latter is exposed to the atmosphere. In the ordinary type ofsteelshell-kiln the heat transmitted to the atmosphere through the shellamounts to at least twelve to fifteen per cent. of the total calorificvalue of the fuel burned in the kiln.

heat, it does take up and return to the kiln via the producer the largerpart of it.

The kiln gas which I introduce into the producer, I withdraw from thekiln at quite a high temperature. By mixing the air, which is at a muchlower temperature than the gas, with the latter, before it reaches thefan or blower, I am able to introduce the draft current into theproducer at the highest tem erature at which it is practicable to handleit with blowers of the ordinary type.

\Vith this invention I can do, what has never, heretofore, beenaccomplished, secure with a. producer fired kiln about as high an outputof. lime per unit of combustible fuel as is secured by the older methodsof burning lime in which the fuel is charged with the limestone. Theadvantage of my method is that where, in the type of kiln referred to,the use of the most expensive fuels, coke of anthracite, is an absolutenecessity, in the process herein revealed I can use the cheaper gradesof fuel, such as slack, lignites, etc. Aside from the saving in fueleffected thereby, the recuperation of the heat of the lime, and theconsequent withdrawal of it in a erable practical value, in that itpermits of its immediate loading for transport as well as lessens thelabor and annoyance of handling it.

already described twodifferent methods are open .to me. In the first, asalreadyv de scribed, I introduce the portion of kiln gas usedformodifying the producer gas by means of a separate blower 4 into the roiducer above the fuel bed or into the con uit between the producerandkiln. In the second method, I introduce into the fuel bed of theproducer, in admixture with the pri- VVhile my device does not save allof this comparatively cool condition, has a consid- In modifying myproducer gas in the wayof operating, the non-conductingproperty maryair, the entire quantity of kiln gas bustible gas so produced, anotherportion of which is required in both the producer and kiln.

The extent to which the carbon dioxid introduced into the fuel bed isdissociated depends upon the temperature of the latter, the thickness ofthe incandescent fuel, and the completeness .and duration of contactbetween the gas and fuel. The latter condition may be said to varyinversely with the velocity of the gaseous current through the fuelb'edl From the above it follows that by regulating the thickness of thefuel, and the velocity of the gaseous current it will be possible tosecure the dissociation of any proportion of the carbon dioxid thatmaybe desired; or, to put it another way, it will be possible to passthrough the producer any quantity of carbon dioxid that we may desireand at the same time secure the dissociation of the pro ortion of thedioxid which I'desire. By ta ing advantage of this fact, I am enabled togreatly simplify the apparatus required by my process and the operationof the same. By properly proportioning my producer andregulating thethickness of the fuel bed, I am enabled to effect the temperaturecontrol of the producer and, at the same time, secure the properdilution of the producer gas, by means of the single'blower, 3. Innormal running, I find it feasible, by maintaining the proper thicknessof fuel bed to operate in the manner just described.

I consists in contacting with 1 To meet abnormal fluctuations in theoperating conditions I have found it desirable to retain the auxiliaryblower, 4. I can thus more quickly vary the quality of the producer gasto meet any abnormal fluctuations without, at the same time, disturbingthe normal working of the producer, than is possible when dependingsolely upon the one blower, 3. v

I .am aware of the process, patented to Eldred, (Letters Patent 795,257,dated July 18, 1905) in which he uses the gases from the calcination oflimestone for the endothermic control of the temperature of the fuel bedin a gas producer, and I do not claim that particular invention.

As hereinbefore pointed out, the primary object ofmy invention is, in aproducer-gas fired-limekiln, the recuperation of the heatof the hotlime. By my present invention I accomplish this by so arranging myapparatus and the circulation of the air and gas currents that themaximum possible ropor tion of the total air required by the kiln andgas-producer is passed in contact with the hot lime.

Having described my invention, what I claim is p l v 1. The process ofcalcining limestone which ited carbon, a portion of the gases produce inthe calcina- .tion of the limestone, mixing with the comthe gases fromthe calcination of the stone, heating air by contacting the same withthe hot lime, and burning said gas mixture with said heated air incontact with said limestone, the portion of calcination gases added tosaid combustible gas, being such that the temperature developed in theburning of said gas is below that at which the impurities in {he stonewill combine chemically with the ime.

2. In the calcination of limestone the process which consists in mixinga portion of the gaseous products of the calcination with air, incontacting such aseous mixture with incandescent carbon orming producergas, in mixing such producer gas with another ortion of the gaseousproducts of the calcination of said limestone, whereby a modifiedproducer as is formed, in contacting the calcined limestone with acurrent of air whereby said air is heated, in mixing such heated air andsaid modified producer gas in contact with the incandescent lime,whereby a calcining flame of regulated intensity is secured, and insubjecting the limestone to calcination by said flame, all substantiallyas described.

3. In the calcination of limestone the process which consists in mixin aportion of the gaseous products of such ca cination with air preheatedby a portion of the sensible heat of said gaseous products, incontacting such aseous mixture with ignited carbonaceous uel, therebyforming producer gas, in mixing such producer gas with another portionof the gaseous products of the calcination of said limestone, whereby amodified producer gas is formed, in contacting the calcined lnnestonewith a current of air whereby said air is heated, and in mixing suchheated air and said modified producer gas in contact with thehighly-heated lime, whereby a calcining flame of regulated intensity issecured, all substantially, as described.

4. In the calcination of limestone, the process which consists in mixinga portion of the gaseous roducts resultin from such calcination with airreheated y a portion of the sensible heat 0 said gaseous products, incontacting such gaseous mixture with incandescent carbonaceous fuel,thereby, formin roducer gas, in contacting the hot calcine imestone witha current of air whereby the said air-current is heated and the saidlime cooled, in modifying the said producer gas by mixing therewithsufiicient of the gaseous products of the calcination to reduce thetemperature of the flame formed on the subsequent combustion of saidproducer gas, be-

low the temperature at which the im urities tact with incandescentmaterial, whereby said gas is burned, and in subjecting the limestone tothe flame of such burning gas, substantially as described.

'5. In the calclnation of limestone the rocess which consists, inwithdrawing at a igh temperature a ortion of the gaseous productsresulting rom such calcination, in contacting such portion of thegaseous products of the calcination with incandescent carbonaceous fuel,suflicient air preheated by a part of the sensible heat of the-gaseousproducts being admixed with said portion of the said products tomaintain said fuel in an ignited condition, whereby a producer gas ofregulated calorific value is formed, in contacting the lime from thecalcina-tion of the limestone with a current of air, whereby said limeis cooled and said air current heated, in mixing said heated air-currentand said producer gas in contact with incandescent material, therebyburning said gas, and in subjecting the said limestone to calcination bythe flame of said burning gas, all, substantially, as described.

6. In the operation of a producer-gas-firedlimekiln, the process, whichconsists in generating the major portion of the producer gas bywithdrawing a portion of the kiln gas, while the same is at a hightemperature, and contacting it with ignited carbonaceous fuel, incontacting the hot lime with a current of air, whereby said lime iscooled and said air-current heated, and in mixing the said heatedair-current, and the producer gas in contact with the hot lime, wherebysaid.

gas is burned and the limestone subjected to a calcinin limestone, thecombination of a furnace for burning said stone, a gas producer, meansflame of moderate intensity, all substantia 1y as described.

the material in said lime-.

for heating air by contact with the shell of said furnace, means forintroducing the soheated air and kiln gas into the fuel bed of saidproducer, means for introducing another ortion of kiln gas into the gasenerated in said producer, means for conucting said gas mixture to.;saidfurnace, and means for burning said gas in contact with the material insaid furnace.

9. In an apparatus for the calcination of limestone, the combination ofa furnace for burning said stone, comprising ,a vertical shaft, havingmeans for charging the limestone and dischargim the kiln gas at the topthereof, means for withdrawing the finished lime from the bottomthereof, meansfor introducing a combustible gas thereinto and burningthe same in contact with the charge in said furnace, and means forheating air by contacting the same with the shell of said v furnace; agas producer;- means for passing into said producer a draft currentcomprised of the said heated air and kiln gas, and

means for conducting the gas from said gas therefrom, means forwithdrawing the fin ished lime from the bottom thereof, means forheating a portion of air by-contacting the same with the shell of saidkiln, means for introducing into the bottom of said'shaft anotherportion of air and heating the same by contact with the finished lime inthe lower part of said kiln, and means for introducing a. combustiblegas into said kiln, means for igniting said gas and means for burningthe-same; a gas producer; means for introducing the first portion ofheated air and kiln gas into said gas producer; and means for conductingthe combustible gas generated in said sproducer to said kiln.

Signed at New ork city in the county of New York and State of hew York.

HENRY L. 'DOHERTY.

Witnesses: Y I i F. D. :TAYLOR, JOHN MCGUIRE.

for the calcination of

